Semiconductor device, power circuit, and computer

ABSTRACT

A semiconductor device according to an embodiment includes a nitride semiconductor layer, an insulating layer provided on the nitride semiconductor layer, a first region provided in the nitride semiconductor layer, and a second region which is provided between the first region in the nitride semiconductor layer and the insulating layer, has a higher electric resistivity than the first region, and includes carbon (C).

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from.Japanese Patent Application No. 2016-144518, filed on Jul. 22, 2016, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor device,a power circuit, and a computer.

BACKGROUND

A semiconductor element, such as a switching element or a diode, is usedin a circuit such as a switching power supply or an inverter. Thesemiconductor element requires a high breakdown voltage and lowon-resistance. The relationship between the breakdown voltage and theon-resistance is a trade-off relationship which is determined bysemiconductor material.

With the progress of technical development, the on-resistance of asemiconductor element is reduced to the limit of silicon which is majorsemiconductor material in use. It is necessary to change thesemiconductor material in order to further improve the breakdown voltageor to further reduce the on-resistance.

A GaN-based semiconductor, such as gallium nitride (GaN) or aluminumgallium nitride (AlGaN), has a higher bandgap than silicon. When theGaN-based semiconductor is used as a switching semiconductor material,it is possible to improve the trade-off relationship determined by thesemiconductor material and to significantly increase the breakdownvoltage or to significantly reduce the on-resistance.

However, for example, a switching element using the GaN-basedsemiconductor has the problem of “current collapse” in which, when ahigh drain voltage is applied, on-resistance increases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view schematically illustrating asemiconductor device according to a first embodiment.

FIG. 2 is a cross-sectional view schematically illustrating thesemiconductor device according to the first embodiment which is beingmanufactured.

FIG. 3 is a cross-sectional view schematically illustrating thesemiconductor device according to the first embodiment which is beingmanufactured.

FIG. 4 is a cross-sectional view schematically illustrating thesemiconductor device according to the first embodiment which is beingmanufactured.

FIG. 5 is a cross-sectional view schematically illustrating thesemiconductor device according to the first embodiment which is beingmanufactured.

FIG. 6 is a cross-sectional view schematically illustrating thesemiconductor device according to the first embodiment which is beingmanufactured.

FIG. 7 is a cross-sectional view schematically illustrating thesemiconductor device according to the first embodiment which is beingmanufactured.

FIG. 8 is a cross-sectional view schematically illustrating thesemiconductor device according to the first embodiment which is beingmanufactured.

FIGS. 9A and 9B are diagrams illustrating the function and effect of thesemiconductor device and a semiconductor device manufacturing methodaccording to the first embodiment.

FIG. 10 is a diagram illustrating the function and effect of thesemiconductor device and the semiconductor device manufacturing methodaccording to the first embodiment.

FIGS. 11A and 11B are diagrams illustrating the function and effect ofthe semiconductor device and the semiconductor device manufacturingmethod according to the first embodiment.

FIG. 12 is a diagram illustrating the function and effect of thesemiconductor device and the semiconductor device manufacturing methodaccording to the first embodiment.

FIGS. 13A and 13B are diagrams illustrating the function and effect ofthe semiconductor device and the semiconductor device manufacturingmethod according to the first embodiment.

FIG. 14 is a diagram illustrating the function and effect of thesemiconductor device and the semiconductor device manufacturing methodaccording to the first embodiment.

FIGS. 15A, 15B, 15C, and 15D are diagrams illustrating the function ofthe semiconductor device manufacturing method according to the firstembodiment.

FIGS. 16A and 16B are diagrams illustrating the function of asemiconductor device according to a second embodiment.

FIGS. 17A and 17B are diagrams illustrating the function of asemiconductor device according to a third embodiment.

FIGS. 18A, 18B, and 18C are diagrams illustrating a semiconductor deviceaccording to a fourth embodiment.

FIG. 19 is a cross-sectional view schematically illustrating asemiconductor device according to a fifth embodiment.

FIG. 20 is a cross-sectional view schematically illustrating asemiconductor device according to a sixth embodiment.

FIG. 21 is a cross-sectional view schematically illustrating asemiconductor device according to a seventh embodiment.

FIG. 22 is a cross-sectional view schematically illustrating asemiconductor device according to an eighth embodiment.

FIG. 23 is a diagram schematically illustrating a computer according toa ninth embodiment.

DETAILED DESCRIPTION

A semiconductor device according to an aspect of the invention includesa nitride semiconductor layer, an insulating layer provided on thenitride semiconductor layer, a first region provided in the nitridesemiconductor layer, and a second region which is provided between thefirst region in the nitride semiconductor layer and the insulatinglayer, has a higher electric resistivity than the first region, andincludes carbon (C).

In the specification, the same or similar members are denoted by thesame reference numerals and the description thereof will not berepeated.

In the specification, a “GaN-based semiconductor” is a general term of asemiconductor including gallium nitride (GaN), aluminum nitride (AlN),indium nitride (InN), and an intermediate composition thereof.

In the specification, the upward direction in the drawings isrepresented by an “upper side” and the downward direction in thedrawings is represented by a “lower side”, in order to indicate thepositional relationship between components. In the specification, theconcept of the “upper side” and the “lower side” does not necessarilyindicate the positional relationship with the direction of gravity.

First Embodiment

A semiconductor device according to this embodiment includes a nitridesemiconductor layer, an insulating layer provided on the nitridesemiconductor layer, a first region provided in the nitridesemiconductor layer, and a second region which is provided between thefirst region in the nitride semiconductor layer and the insulatinglayer, has a higher electric resistivity than the first region, andincludes carbon (C).

In addition, a semiconductor device according to this embodimentincludes a nitride semiconductor layer, an insulating layer provided onthe nitride semiconductor layer, and a region which is provided in aninsulating-layer-side portion of the nitride semiconductor layer andincludes an atom X that is an atom other than a nitrogen atom forming acrystal structure of the nitride semiconductor layer, forms a bond witha carbon atom, and has a dangling bond.

In the above-mentioned structure of the semiconductor device accordingto this embodiment, it is possible to reduce a level (state) that ispresent in the vicinity of the interface between the nitridesemiconductor layer and the insulating layer and becomes an electrontrap. Therefore, it is possible to prevent current collapse caused bythe electron trap.

Hereinafter, an example in which, when atoms other than nitrogen atomsforming the crystal structure of the nitride semiconductor layer areatoms X, the atoms X are gallium (Ga) atoms or the atoms X are gallium(Ga) atoms and aluminum (Al) atoms will be described. That is, anexample in which the nitride semiconductor layer is made of galliumnitride or aluminum gallium nitride will be described.

FIG. 1 is a cross-sectional view schematically illustrating thesemiconductor device according to this embodiment. The semiconductordevice according to this embodiment is a high electron mobilitytransistor (HEMT) using a GaN-based semiconductor.

As illustrated in FIG. 1, an HEMT (semiconductor device) 100 includes asubstrate 10, a buffer layer 12, a channel layer (first semiconductorregion) 14, a barrier layer (second semiconductor region) 16, a sourceelectrode (first electrode) 18, a drain electrode (second electrode) 20,an insulating layer 22, a p-type layer 24, and a gate electrode 28.

The channel layer (first semiconductor region) 14 and the barrier layer(second semiconductor region) 16 are nitride semiconductor layers. Thebarrier layer 16 includes a low-resistance region (first region) 16 aand a high-resistance region (a second region or a region) 16 b.

The substrate 10 is made of, for example, silicon (Si). In addition tosilicon, for example, sapphire (Al₂O₃) or silicon carbide (SiC) may beapplied to the substrate 10.

The buffer layer 12 is provided on the substrate 10. The buffer layer 12has a function of reducing the lattice mismatch between the substrate 10and the channel layer 14. The buffer layer 12 has a multi-layerstructure of, for example, aluminum gallium nitride (Al_(W)Ga_(1-W)N(0<W<1)).

The channel layer 14 is provided on the buffer layer 12. The channellayer 14 is also referred to as an electron transit layer. The channellayer 14 is made of, for example, Al_(X)Ga_(1-X)N (0≤X<1). Specifically,the channel layer 14 is made of, for example, GaN. The thickness of thechannel layer 14 is, for example, equal to or greater than 0.1 μm andequal to or less than 10 μm.

The barrier layer 16 is provided on the channel layer 14. The barrierlayer 16 is also referred to as an electron supply layer. The bandgap ofthe barrier layer 16 is greater than the bandgap of the channel layer14. The barrier layer 16 is made of, for example, Al_(Y)Ga_(1-Y)N(0<Y≤1, X<Y). Specifically, the barrier layer 16 is made of, forexample, Al_(0.25)Ga_(0.75)N. The thickness of the barrier layer 16 is,for example, equal to or greater than 10 nm and equal to or less than100 nm.

The interface between the channel layer 14 and the barrier layer 16 is ahetero-junction interface. A two-dimensional electron gas (2 DEG) isformed at the hetero-junction interface of the HEMT 100 and becomes acarrier.

The barrier layer 16 includes the low-resistance region 16 a and thehigh-resistance region 16 b. The high-resistance region 16 b is providedin the barrier layer 16 so as to be close to or adjacent to theinsulating layer 22. For example, the high-resistance region 16 b comesinto contact with the insulating layer 22.

The electric resistivity of the high-resistance region 16 b is higherthan the electric resistivity of the low-resistance region 16 a. Themagnitude relationship between the electric resistivities can bedetermined by, for example, spreading resistance analysis (SRA) orscanning spreading resistance microscopy (SSRM).

The electric resistivity of the low-resistance region 16 a is reducedsince carrier concentration is higher than that of the high-resistanceregion 16 b. Therefore, the magnitude relationship between the electricresistivities can be determined by, for example, scanning capacitancemicroscopy (SCM) that can determine the magnitude of carrierconcentration.

For example, there is a nitrogen defect (hereinafter, also referred toas VN) in aluminum gallium nitride in the low-resistance region 16 a.The nitrogen defect functions as a donor. Therefore, the nitrogen defectcauses aluminum gallium nitride to change to an n type. As a result, theelectric resistivity of the low-resistance region 16 a is reduced.

The high-resistance region 16 b includes one carbon atom that is presentat the lattice position of a nitrogen atom. For example, one carbon atomis introduced to the lattice position of a nitrogen atom in aluminumgallium nitride. The one carbon atom introduced to the lattice positionof the nitrogen atom function as an acceptor.

The carrier concentration of the high-resistance region 16 b is lowerthan that of the low-resistance region 16 a due to the interactionbetween VN functioning as a donor and the carbon atom functioning as anacceptor. Therefore, the electric resistivity of the high-resistanceregion 16 b is higher than that of the low-resistance region 16 a.

In the high-resistance region 16 b, VN and the carbon atom are adjacentto each other. In the high-resistance region 16 b, VN and the carbonatom are so close that they electrically interact with each other.

In the high-resistance region 16 b, it is preferable that the amount ofcarbon atoms be substantially equal to the amount of VN. Here, that theamount of carbon atoms is substantially equal to the amount of VN means,for example, that the amount of VN is equal to or greater than 0.8 timesthe amount of carbon atoms and equal to or less than 1.2 times theamount of carbon atoms.

It is assumed that an atom other than a nitrogen atom forming thebarrier layer 16 is an atom X. In this case, in the HEMT 100, thehigh-resistance region 16 b includes the atom X that forms a bond with acarbon atom and has a dangling bond. Since the atom X forms a bond withthe carbon atom and has a dangling bond, a structure in which the carbonatom and VN are closest to each other is achieved. In other words, thecarbon atom, the atom X, and VN form a complex.

The bond between the atom X and the carbon atom, the dangling bond ofthe atom X, and a complex of the carbon atom, the atom X, and VN in thehigh-resistance region 16 b can be measured by, for example, X-rayphotoelectron spectroscopy (XPS), infrared spectroscopy, or Ramanspectroscopy.

When the carbon atom and VN form a complex, the carbon atom and VN havethe same spatial distribution.

The carbon concentration of the high-resistance region 16 b is, forexample, equal to or greater than 1×10¹⁹ cm⁻³. The carbon concentrationof the high-resistance region 16 b can be measured by, for example,secondary ion mass spectroscopy (SIMS).

For example, when the atom X is a gallium (Ga) atom, one gallium atom isbonded to one carbon atom and one gallium atom has a dangling bond. Whenthe atom X is an aluminum (Al) atom, one aluminum atom is bonded to onecarbon atom and one aluminum atom has a dangling bond.

The thickness of the high-resistance region 16 b is, for example, equalto or greater than 0.5 nm and equal to or less than 10 nm. The electricresistivity of the high-resistance region 16 b is not necessary beuniform. The electric resistivity may have variation in thehigh-resistance region 16 b.

The insulating layer 22 is provided on the high-resistance region 16 b.The insulating layer 22 functions as a gate insulating layer of the HEMT100.

The insulating layer 22 is made of, for example, silicon oxide. Theinsulating layer 22 may be made of, for example, silicon nitride orsilicon oxynitride. In addition, the insulating layer 22 may be, forexample, a stacked structure of materials selected from silicon oxide,silicon nitride, and silicon oxynitride.

The thickness of the insulating layer 22 is, for example, equal to orgreater than 10 nm and equal to or less than 100 nm.

The source electrode 18 and the drain electrode 20 are formed on thebarrier layer 16. The source electrode 18 and the drain electrode 20come into contact with the low-resistance region 16 a.

The source electrode 18 and the drain electrode 20 are, for example,metal electrodes. The metal electrode is, for example, a stackedstructure of titanium (Ti) and aluminum (Al).

It is preferable that the source electrode 18 and the drain electrode 20come into ohmic contact with the barrier layer 16. Since the sourceelectrode 18 and the drain electrode 20 come into contact with thelow-resistance region 16 a, it is possible to achieve an ohmic contact.

The distance between the source electrode 18 and the drain electrode 20is, for example, equal to or greater than 5 μm and equal to or less than30 μm.

The p-type layer 24 is provided on the insulating layer 22 between thesource electrode 18 and the drain electrode 20. The p-type layer 24 hasa function of increasing the threshold voltage of the HEMT 100. Sincethe p-type layer 24 is provided, the HEMT 100 can operate as anormally-off transistor.

The p-type layer 24 is made of, for example, p-type gallium nitride(GaN) to which magnesium (Mg) is applied as p-type impurities. Thep-type layer 24 is, for example, polycrystalline.

The gate electrode 28 is provided on the p-type layer 24. The gateelectrode 28 is, for example, a metal electrode. The gate electrode 28is made of, for example, titanium nitride (TiN).

Next, an example of a method for manufacturing the semiconductor deviceaccording to this embodiment will be described. FIGS. 2 to 8 arecross-sectional views schematically illustrating the semiconductordevice according to this embodiment which is being manufactured.

First, the substrate 10, for example, a Si substrate is prepared. Then,for example, the buffer layer 12 is grown on the Si substrate byepitaxial growth.

The buffer layer 12 is, for example, a multi-layer structure of aluminumgallium nitride (Al_(W)Ga_(1-W)N (0<W<1)). For example, the buffer layer12 is grown by a metal organic chemical vapor deposition (MOCVD) method.

Then, gallium nitride which will be the channel layer 14 and aluminumgallium nitride which will be the barrier layer 16 are formed on thebuffer layer 12 by epitaxial growth (FIG. 2). The aluminum galliumnitride has, for example, a composition of Al_(0.25)Ga_(0.75)N. Forexample, the channel layer 14 and the barrier layer 16 are grown by theMOCVD method.

Then, an insulating layer 21 is formed on the barrier layer 16 (FIG. 3).The insulating layer 21 is, for example, a silicon oxide layer, asilicon oxynitride layer, or a silicon nitride layer which is formed bya CVD method. The insulating layer 21 is a through film which is used inthe subsequent carbon ion implantation process.

Then, carbon ions are implanted into the barrier layer 16 through theinsulating layer 21 (FIG. 4). For example, an accelerating voltage isset such that the projected range (RP) of the carbon ions is in thevicinity of the interface between the barrier layer 16 and theinsulating layer 21. Carbon is also included in the insulating layer 21.

Then, a heat treatment is performed in a non-oxidizing atmosphere. Theheat treatment temperature is, for example, equal to or greater than900° C. and equal to or less than 1200° C. The heat treatment atmosphereis, for example, a nitrogen atmosphere or an argon atmosphere.

The high-resistance region 16 b is formed in a portion of the barrierlayer 16 which is close to the insulating layer 21 by the heat treatment(FIG. 5). A portion of the barrier layer 16 below the high-resistanceregion 16 b becomes the low-resistance region 16 a having a lowerelectric resistivity than the high-resistance region 16 b.

One carbon atom is introduced to the lattice position of a nitrogen atomin the barrier layer 16 to form the high-resistance region 16 b. Forexample, one carbon atom is introduced to a nitrogen defect of thebarrier layer 16.

For example, gallium or aluminum in the barrier layer 16 is diffusedinto the insulating layer 21. During the heat treatment, VN is generatedin the barrier layer 16. Nitrogen which has been generated during theheat treatment is also diffused into the insulating layer 21.

Then, the insulating layer 21 is removed by wet etching (FIG. 6). Theinsulating layer 21 is removed under the condition that thehigh-resistance region 16 b remains. Impurities, such as nitrogen,carbon, gallium, or aluminum, included in the insulating layer 21 areremoved together with the insulating layer 21.

Then, the insulating layer 22 is formed on the barrier layer 16 (FIG.7). The insulating layer 22 is, for example, a silicon oxide layer whichis formed by the CVD method.

Then, the p-type layer 24 and the gate electrode 28 are formed on theinsulating layer 22 (FIG. 8).

Then, the source electrode 18 and the drain electrode 20 are formed onthe barrier layer 16. When the source electrode 18 and the drainelectrode 20 are formed, the high-resistance region 16 b is removed byetching.

The source electrode 18 and the drain electrode 20 are formed so as tocome into contact with the low-resistance region 16 a. The sourceelectrode 18 and the drain electrode 20 are formed, with the gateelectrode 28 interposed therebetween.

The HEMT 100 illustrated in FIG. 1 is formed by the above-mentionedmanufacturing method.

Next, the function and effect of the semiconductor device and thesemiconductor device manufacturing method according to this embodimentwill be described. FIGS. 9A, 9B, 10, 11A, 11B, 12, 13A, 13B, and 14 arediagrams illustrating the function of the semiconductor device and thesemiconductor device manufacturing method according to this embodiment.Hereinafter, an example in which gallium nitride is used as the nitridesemiconductor will be described.

FIGS. 9A and 9B and FIG. 10 are diagrams illustrating a nitrogen defect(VN). FIG. 9A is a diagram schematically illustrating VN in galliumnitride. FIG. 9B is a diagram illustrating the level formed by VN whichis calculated by first principle calculation. FIG. 10 is a band diagramillustrating gallium nitride that includes VN calculated by the firstprinciple calculation.

As illustrated in FIG. 9A, VN in gallium nitride is formed by theseparation of a nitrogen atom from the gallium nitride. A gallium atomadjacent to VN has a dangling bond. VN functions as a donor in thegallium nitride.

The first principle calculation proves that a donor level is formed inthe bandgap of gallium nitride by VN, as illustrated in FIGS. 9B and 10.In FIG. 9B, a black circle indicates a state in which the level isfilled with an electron and a white circle indicates a state in whichthe level is not filled with an electron.

In the HEMT, it is considered that one of the causes of current collapseis a change in the density of 2 DEG caused by the trapping of anelectron at the level of the bandgap. In general, VN is present ingallium nitride. It is considered that the density of VN is particularlyhigh in the vicinity of the interface between the barrier layer 16 andthe insulating layer 22. In addition, the dangling bond of a galliumatom is present at the interface between the barrier layer 16 and theinsulating layer 22.

For example, when an electron is trapped at the level formed by VN, VNis negatively charged. Therefore, for example, the density of 2 DEGimmediately below VN is reduced. As a result, current collapse occurs.

When an electron is trapped in VN immediately below the gate electrode28, the threshold voltage of the HEMT 100 varies.

The dangling bond of the gallium atom that is present at the interfacebetween the barrier layer 16 and the insulating layer 22 forms the samelevel as VN. Therefore, the function generated by the dangling bond ofthe gallium atom that is present at the interface between the barrierlayer 16 and the insulating layer 22 is the same as that generated byVN.

FIGS. 11A and 11B and FIG. 12 are diagrams illustrating one carbon atomthat is present at the lattice position of a nitrogen atom. FIG. 11A isa diagram schematically illustrating one carbon atom that is present atthe lattice position of a nitrogen atom in gallium nitride. FIG. 11B isa diagram illustrating the level formed by the carbon atom which iscalculated by the first principle calculation. FIG. 12 is a band diagramillustrating gallium nitride that includes the carbon atom calculated bythe first principle calculation.

As illustrated in FIG. 11A, one carbon atom is introduced to VN ingallium nitride. Therefore, one carbon atom that is present at thelattice position of a nitrogen atom in gallium nitride is formed. Thecarbon atom functions as an acceptor in the gallium nitride.

The first principle calculation proves that the carbon atom introducedto the VN forms an acceptor level in the bandgap of gallium nitride asillustrated in FIGS. 11B and 12. In FIG. 11B, a black circle indicates astate in which the level is filled with an electron and a white circleindicates a state in which the level is not filled with an electron.

FIGS. 13A, 13B, and 14 are diagrams illustrating a case in which VN anda carbon atom at the position of a nitrogen atom coexist. FIG. 13A is adiagram illustrating VN and a nitrogen atom in gallium nitride. FIG. 13Bis a diagram illustrating a level when a carbon atom and VN calculatedby the first principle calculation coexist.

FIG. 13A illustrates a state in which VN and a carbon atom are closestto each other when VN and the carbon atom coexist in gallium nitride. Inother words, FIG. 13A illustrates a state in which one gallium atom hasa dangling bond and is bonded to a carbon atom. The carbon atom, theatom X, and VN form a complex.

The first principle calculation proves that, when VN and a carbon atomcoexist, a structure in which an electron moves from the donor level ofVN to the acceptor level of the carbon atom is stabilized, asillustrated in FIG. 13B. At that time, the level formed by VN is movedto a conduction band and the level formed by the carbon atom is moved toa valence band. Therefore, the level in the bandgap of gallium nitrideis removed, as illustrated in FIGS. 13B and 14.

In this embodiment, a carbon atom is provided at the position of anitrogen atom in gallium nitride. The level in the bandgap of galliumnitride is removed by the interaction between VN and the carbon atom atthe position of a nitrogen atom and the interaction between the carbonatom at the position of a nitrogen atom and a dangling bond at theinterface between the barrier layer 16 and the insulating layer 22.Therefore, electron trap is prevented. As a result, it is possible toprevent current collapse.

The removal of the level in the bandgap of gallium nitride can bedetermined by, for example, deep level transient spectroscopy (DLTS).

When VN and the carbon atom at the position of a nitrogen atom coexist,the donor level and the acceptor level are removed. As a result,carriers are cancelled and carrier concentration is reduced. Therefore,the electric resistivity of gallium nitride increases.

Since electron trap immediately below the gate electrode 28 isprevented, a variation in the threshold voltage is prevented.

FIGS. 15A, 15B, 15C, and 15D are diagrams illustrating the function ofthe semiconductor device manufacturing method according to thisembodiment.

It is assumed that VN illustrated in FIG. 15A is present in galliumnitride. As illustrated in FIG. 15B, one carbon atom is introduced intogallium nitride by carbon ion implantation.

As illustrated in FIG. 15C, a carbon atom is introduced to VN and islocated at the lattice position of a nitrogen atom.

According to the first principle calculation, a carbon atom and VN areclose to each other and the energy of the system is reduced. As aresult, a stable structure is formed. Therefore, as illustrated in FIG.15D, a structure in which a nitrogen atom and VN are close to each otheris formed by the heat treatment. Specifically, for example, a structurein which one gallium atom has a dangling bond and is bonded to onecarbon atom is formed. In other words, a complex of the carbon atom, thegallium atom, and VN is formed.

It is preferable that the heat treatment be performed at a hightemperature after carbon ion implantation, in order to form the complexof the carbon atom, the gallium atom, and VN. The heat treatmenttemperature is preferably equal to or greater than 900° C. and equal toor less than 1200° C. and more preferably equal to or greater than 1000°C.

VN is formed in gallium nitride in order to increase entropy. Forexample, it is assumed that the amount of carbon more than the amount ofVN in gallium nitride is introduced by carbon ion implantation. In thiscase, theoretically, since the same amount of VN as the amount of carbonintroduced forms a complex, no VN is independently present. When theheat treatment is performed for a long time, the same amount of VN asthe remaining amount of carbon is newly generated in order to form acomplex. The new VN forms a complex to stabilize the system. Therefore,the amount of carbon atoms or VN which remains independently in galliumnitride without forming a complex is very small.

For example, it is assumed that the amount of carbon less than theamount of VN in gallium nitride is introduced by carbon ionimplantation. In this case, theoretically, the same amount of VN as theamount of carbon introduced forms a complex. Surplus VN remains in thegallium nitride. There is a concern that current collapse and avariation in the threshold voltage will occur due to the donor level inthe bandgap formed by the remaining VN.

After a sufficient amount of carbon is introduced, the heat treatment isperformed at a high temperature for a sufficient period of time to formthe high-resistance region 16 b. In other words, when a sufficientthermal budget is applied after a sufficient amount of carbon isintroduced, it is possible to form the high-resistance region 16 b. Whena sufficient amount of carbon is not introduced or when a sufficientthermal budget is not applied by the heat treatment, it is difficult toform the high-resistance region 16 b.

In the manufacturing method according to this embodiment, after the heattreatment, the insulating layer 21 which is used as a through film forcarbon ion implantation is removed. When the insulating layer 21 is madeof silicon oxide, nitrogen is diffused into the insulating layer 21 bythe heat treatment and silicon oxide is changed to silicon oxynitride.

After the insulating layer 21 is removed, the heat treatment may beperformed. In this case, nitrogen is effectively emitted from thebarrier layer 16, which makes it easy to form VN.

In addition to the silicon oxide, silicon oxynitride or silicon nitridemay be used for the insulating layer 21 which is a through film. Inparticular, when silicon nitride with high density is used, carbon ionblocking capability during ion implantation is improved. Therefore, itis easy to set an accelerating voltage such that the projected range(RP) of carbon ions is located in the vicinity of the interface betweenthe barrier layer 16 and the insulating layer 21.

It is considered that the concentration of VN in gallium nitride isequal to or greater than about 1×10¹⁸ cm⁻³ and equal to or less thanabout 1×10¹⁹ cm⁻³. The carbon concentration of the high-resistanceregion 16 b is preferably equal to or greater than 1×10¹⁹ cm⁻³, morepreferably equal to or greater than 1×10²⁰ cm⁻³, and most preferablyequal to or greater than 1×10²¹ cm⁻³, in order to prevent the generationof surplus VN.

As described above, since the amount of carbon is more than the amountof VN in gallium nitride in an initial stage, it is possible to controlthe level in the bandgap. When the heat treatment is performed at a hightemperature for a sufficiently long period of time after a sufficientamount of carbon is introduced, the amount of carbon and the amount ofVN are equal to each other and have the same distribution, which makesit easy to form the high-resistance region 16 b.

The manufacturing method according to this embodiment has the followingcharacteristics (1) to (3). (1) The amount of carbon increases and VN isadditionally formed by the heat treatment. At that time, it ispreferable that the heat treatment be performed at a sufficiently hightemperature for a sufficiently long period of time, that is, asufficient thermal budget be applied. (2) The insulating layer 21 usedas a through film is removed after the heat treatment for forming thehigh-resistance region 16 b or before the heat treatment. In the formercase, nitrogen which is emitted from the barrier layer 16 and is thenintroduced into the insulating layer 21 is removed. In the latter case,the emission efficiency of nitrogen from the barrier layer 16 isimproved. (3) Since the insulating layer 21 is used as a through filmduring carbon ion implantation, it is easy to set an acceleratingvoltage such that the projected range (RP) of carbon ions is located inthe vicinity of the interface between the barrier layer 16 and theinsulating layer 21. When the through film is not provided, it isdifficult to introduce carbon into the vicinity of the surface of thebarrier layer 16, for example, a region that is about several nanometersaway from the surface. For example, since RP is located in the throughfilm, it is possible to introduce carbon only to the vicinity of thesurface of the barrier layer 16. For example, when silicon nitride withhigh density is used for the insulating layer 21, carbon can beintroduced only to the vicinity of the surface of the barrier layer 16with a steep profile by ion implantation. Since the through film isremoved, surplus carbon introduced by ion implantation does not remainafter the semiconductor device is completed.

In the manufacturing method according to this embodiment, after carbonions are implanted, the insulating layer 21 is removed by wet etching.Impurities, such as carbon, gallium, or aluminum, included in theinsulating layer 21 are removed together with the insulating layer 21.Therefore, charge trap at the level formed by impurities, such asnitrogen, carbon, gallium, or aluminum, included in the insulating layer21 is prevented. As a result, it is possible to prevent, for example,current collapse, a variation in the threshold voltage, and an increasein leakage current caused by charge trap in the insulating layer 21.

FIGS. 9A, 9B, 10, 11A, 11B, 12, 13A, 13B, 14, 15A, 15B, 15C, and 15Dillustrate an example in which the atom X other than a nitrogen atomforming the nitride semiconductor layer is a gallium atom. That is, anexample in which the nitride semiconductor layer is made of galliumnitride has been described. However, in the other GaN-basedsemiconductors in which some or all of the gallium atoms in galliumnitride are substituted with aluminum or indium atoms, the same functionas that generated in gallium nitride is generated. That is, even whenthe atom X is an aluminum or indium atom, the same function as that whenthe atom X is a gallium atom is generated.

In this embodiment, when a carbon atom is formed at the position of anitrogen atom, the heat treatment temperature is preferably equal to orgreater than 900° C. and more preferably equal to or greater than 1000°C. Since the heat treatment is performed at a high temperature, it ispossible to reduce the distance between VN and a carbon atom to a valueat which VN and the carbon atom interact with each other.

According to the semiconductor device and the semiconductor devicemanufacturing method of this embodiment, since the level in the bandgapis reduced, it is possible to prevent current collapse. In addition,since the level in the bandgap is reduced, it is possible to prevent avariation in the threshold voltage. Therefore, it is possible to achievea semiconductor device with high reliability.

Second Embodiment

A semiconductor device according to this embodiment differs from thesemiconductor device according to the first embodiment in that thesecond region includes at least one element selected from the groupconsisting of oxygen (O), sulfur (S), selenium (Se), and tellurium (Te).

In addition, the semiconductor device according to this embodimentdiffers from the semiconductor device according to the first embodimentin that it includes an atom X which forms a bond with a carbon atom andforms a bond with a first atom of at least one element selected from thegroup consisting of oxygen (O), sulfur (S), selenium (Se), and tellurium(Te).

The semiconductor device according to this embodiment differs from thesemiconductor device according to the first embodiment in that itincludes a first atom at the position of a nitrogen atom, instead of VNin the first embodiment. Hereinafter, the description of the samecontent as that in the first embodiment will not be repeated.

The high-resistance region 16 b illustrated in FIG. 1 includes onecarbon atom at the lattice position of a nitrogen atom. For example, onecarbon atom is introduced to the lattice position of a nitrogen atom inaluminum gallium nitride. The one carbon atom introduced to the latticeposition of a nitrogen atom functions as an acceptor.

In addition, the high-resistance region 16 b includes a first atom of atleast one element selected from the group consisting of oxygen (O),sulfur (S), selenium (Se), and tellurium (Te). The first atom is presentat the lattice position of a nitrogen atom. The first atom functions asa donor.

The carrier concentration of the high-resistance region 16 b is lowerthan that of the low-resistance region 16 a due to the interactionbetween the first atom functioning as a donor and the carbon atomfunctioning as an acceptor. Therefore, the electric resistivity of thehigh-resistance region 16 b is higher than that of the low-resistanceregion 16 a.

In the high-resistance region 16 b, the first atom and the carbon atomare close to each other. In the high-resistance region 16 b, the firstatom and the carbon atom are so close that they electrically interactwith each other.

It is assumed that an atom other than a nitrogen atom forming thebarrier layer 16 is an atom X. In this case, the high-resistance region16 b includes the atom X that forms a bond with a carbon atom and formsa bond with the first atom. Since the atom X forms a bond with thecarbon atom and forms a bond with the first atom, a structure in whichthe carbon atom and the first atom are closest to each other isachieved. In other words, the carbon atom, the atom X, and the firstatom form a complex.

The bond between the atom X and the carbon atom, the bond between theatom X and the first atom, and a complex of the carbon atom, the atom X,and the first atom in the high-resistance region 16 b can be measuredby, for example, X-ray photoelectron spectroscopy, infraredspectroscopy, or Raman spectroscopy.

The carbon concentration of the high-resistance region 16 b is, forexample, equal to or greater than 1×10¹⁹ cm⁻³. The concentration of thefirst atom in the high-resistance region 16 b is, for example, equal toor greater than 1×10¹⁹ cm⁻³. The concentration of carbon and the firstatom in the high-resistance region 16 b can be measured by, for example,secondary ion mass spectroscopy.

For example, the semiconductor device according to this embodiment canbe manufactured by implanting the first atoms into the barrier layer 16,using ion implantation, at the same time as carbon ions are implantedinto the barrier layer 16 in the semiconductor device manufacturingmethod according to the first embodiment.

For example, when the atom X is a gallium (Ga) atom and the first atomis an oxygen (O) atom, one gallium atom is bonded to one carbon atom andthe same gallium atom is bonded to one oxygen atom. When the atom X isan aluminum (Al) atom, one aluminum atom is bonded to one carbon atomand the same aluminum atom is bonded to one oxygen atom.

FIGS. 16A and 16B are diagrams illustrating the function of thesemiconductor device according to this embodiment. FIG. 16A is a diagramschematically illustrating an oxygen atom and a carbon atom in galliumnitride. FIG. 16B is a diagram illustrating a level when the carbon atomand the oxygen atom calculated by the first principle calculationcoexist.

As illustrated in FIG. 16A, an oxygen atom in gallium nitride is presentat the lattice position of a nitrogen atom in gallium nitride. A carbonatom in gallium nitride is present at the lattice position of a nitrogenatom in gallium nitride. FIG. 16A illustrates a state in which an oxygenatom and a carbon atom are closest to each other when the oxygen atomand the carbon atom coexist in gallium nitride. In other words, onegallium atom is bonded to an oxygen atom and the same gallium atom isbonded to a carbon atom. The carbon atom, the atom X, and the oxygenatom form a complex.

The first principle calculation proves that, when an oxygen atom and acarbon atom coexist, a structure in which an electron moves from thedonor level of the oxygen atom to the acceptor level of the carbon atomis stabilized, as illustrated in FIG. 16B. At that time, the levelformed by the oxygen atom is moved to a conduction band and the levelformed by the carbon atom is moved to a valence band. Therefore, thelevel in the bandgap of gallium nitride is removed, as illustrated inFIG. 16B.

In this embodiment, an oxygen atom is provided at the position of anitrogen atom and a carbon atom is provided at the position of anitrogen atom in gallium nitride. The level in the bandgap of galliumnitride is removed by the interaction between the oxygen atom and thecarbon atom. Therefore, electron trap is prevented. As a result, it ispossible to prevent current collapse.

The removal of the level in the bandgap of gallium nitride can bedetermined by, for example, DLTS.

When an oxygen atom and a carbon atom coexist, the donor level and theacceptor level are removed. As a result, carriers are cancelled andcarrier concentration is reduced. Therefore, the electric resistivity ofgallium nitride increases.

Since electron trap immediately below the gate electrode 28 isprevented, a variation in the threshold voltage is prevented.

VN is formed in gallium nitride in order to increase entropy. When acomplex of a carbon atom, a gallium atom, and an oxygen atom is formedby carbon and oxygen ion implantation and a sufficient heat treatment,entropy increases. Therefore, theoretically, when the amount of complexis sufficient, VN is removed. When VN remains, there is a concern thatcurrent collapse and a variation in the threshold voltage will occur dueto the donor level in the bandgap caused by the remaining VN.

For example, it is assumed that the amount of carbon more than theamount of VN in gallium nitride is introduced by carbon ionimplantation. At that time, it is assumed that the amount of oxygen morethan the amount of carbon is introduced by ion implantation. In thiscase, theoretically, VN is removed. However, surplus oxygen remains.There is a concern that current collapse and a variation in thethreshold voltage will occur due to the donor level in the bandgapcaused by the remaining oxygen.

For example, it is assumed that the amount of carbon more than theamount of VN in gallium nitride is introduced by carbon ionimplantation. At that time, it is assumed that the amount of oxygen lessthan the amount of carbon is introduced by ion implantation. Asufficient amount of carbon is introduced such that the amount of carbonimplanted is more than the sum of the amount of VN in gallium nitrideand the amount of oxygen implanted.

In this case, when the amount of oxygen is more than the amount of VN,VN is filled with oxygen and is removed. Then, the same amount of VN asthe amount of carbon which remains without forming a complex with oxygenis generated in order to form a complex with carbon. In other words, VNfor compensating for surplus carbon is newly generated. Therefore, theamount of carbon atoms, oxygen atoms, or VN which remains independentlyin gallium nitride is very small.

In this case, when the amount of oxygen is less than the amount of VN, asufficiently large amount of carbon is introduced. Therefore, surplus VNforms a complex with a carbon atom and no VN remains independently.Then, the same amount of VN as the amount of carbon which remainswithout forming a complex with oxygen is generated in order to form acomplex with carbon. Therefore, the amount of carbon atoms, oxygenatoms, or VN which remains independently in gallium nitride is verysmall.

For example, it is assumed that the amount of carbon less than theamount of VN in gallium nitride is introduced by carbon ionimplantation. At that time, it is assumed that the amount of oxygen morethan the amount of carbon is introduced by ion implantation. In thiscase, theoretically, VN corresponding to a shortage of carbon remainsindependently. In addition, surplus oxygen remains independently. Thereis a concern that current collapse and a variation in the thresholdvoltage will occur due to the donor level in the bandgap caused by VNand oxygen which remain independently.

For example, it is assumed that the amount of carbon less than theamount of VN in gallium nitride is introduced by carbon ionimplantation. At that time, it is assumed that the amount of oxygen lessthan the amount of carbon is introduced by ion implantation. In thiscase, no oxygen remains independently and VN corresponding to a shortagein carbon remains independently. There is a concern that currentcollapse and a variation in the threshold voltage will occur due to thedonor level in the bandgap caused by VN which remains independently.

Therefore, it is preferable that the concentration of carbon in galliumnitride be higher than the concentration of VN before carbon or oxygenis introduced, in order to prevent current collapse and a variation inthe threshold voltage. In addition, it is preferable that theconcentration of carbon in gallium nitride be higher than theconcentration of oxygen in gallium nitride.

It is considered that the concentration of VN in gallium nitride isequal to or greater than about 1×10¹⁸ cm⁻³ and equal to or less thanabout 1×10¹⁹ cm⁻³. The carbon concentration of the high-resistanceregion 16 b is preferably equal to or greater than 1×10¹⁹ cm⁻³, morepreferably equal to or greater than 1×10²⁰ cm⁻³, and most preferablyequal to or greater than 1×10²¹ cm⁻³, in order to prevent the generationof surplus VN. Since the amount of carbon is more than the amount of VNin gallium nitride, it is possible to control the level in the bandgap.

In this embodiment, instead of the oxygen atom, an atom of sulfur (S),selenium (Se), or tellurium (Te) which is a group 16 element and abivalent element may be used as the first atom. In this case, the sameeffect as that when the first atom is an oxygen atom is obtained.

An energy gain obtained when a carbon atom and the first atom (O, S, Se,or Te) are close to each other is higher than that obtained when acarbon atom and VN are close to each other and high stability isobtained. The reason is that, when atoms are close to each other, it ispossible to ensure the localization of charge and coulomb interaction isstrong, as compared to when an atom and VN are close to each other.

The amount of first atoms or the distribution thereof can be controlledby ion implantation conditions. Therefore, it is easy to perform controlsuch that the amount of carbon is substantially equal to the amount offirst atoms or the distribution of carbon is equal to the distributionof the first atoms. Similarly to the first embodiment, for the firstatom, a sufficient amount of carbon is introduced and a shortage of thefirst atoms can be supplemented with VN generated by a high-temperatureheat treatment.

According to the semiconductor device and the semiconductor devicemanufacturing method of this embodiment, it is possible to preventcurrent collapse, similarly to the first embodiment. In addition, it ispossible to achieve a semiconductor device with high reliability. It ispossible to achieve a semiconductor device with a higher stability thanthat in the first embodiment. Furthermore, controllability duringmanufacture is higher than that in the first embodiment.

Third Embodiment

A semiconductor device according to this embodiment differs from thesemiconductor device according to the first embodiment in that thesecond region includes at least one element selected from the groupconsisting of silicon (Si), germanium (Ge), titanium (Ti), zirconium(Zr), hafnium (Hf), and iron (Fe).

The semiconductor device according to this embodiment differs from thesemiconductor device according to the first embodiment in that itincludes an atom X which forms a bond with a carbon atom and forms abond with a second atom of one element selected from the groupconsisting of silicon (Si), germanium (Ge), titanium (Ti), zirconium(Zr), hafnium (Hf), and iron (Fe).

The semiconductor device according to this embodiment differs from thesemiconductor device according to the first embodiment in that itincludes the second atom at the position of the atom X, instead of VN inthe first embodiment. Hereinafter, the description of the same contentas that in the first embodiment will not be repeated.

The high-resistance region 16 b illustrated in FIG. 1 includes onecarbon atom at the lattice position of a nitrogen atom. For example, onecarbon atom is introduced to the lattice position of a nitrogen atom inaluminum gallium nitride. The one carbon atom introduced to the latticeposition of a nitrogen atom functions as an acceptor.

When an atom other than a nitrogen atom forming the barrier layer 16 isan atom X, the high-resistance region 16 b includes a second atom of atleast one element selected from the group consisting of hafnium (Hf),silicon (Si), germanium (Ge), titanium (Ti), zirconium (Zr), and iron(Fe). The second atom is present at the lattice position of the atom X.The second atom functions as a donor.

The carrier concentration of the high-resistance region 16 b is lowerthan that of the low-resistance region 16 a due to the interactionbetween the second atom functioning as a donor and the carbon atomfunctioning as an acceptor. Therefore, the electric resistivity of thehigh-resistance region 16 b is higher than that of the low-resistanceregion 16 a.

In the high-resistance region 16 b, the second atom and the carbon atomare close to each other. In the high-resistance region 16 b, the secondatom and the carbon atom are so close that they electrically interactwith each other.

It is assumed that an atom other than a nitrogen atom forming thebarrier layer 16 is the atom X. The second atom substitutes the atom Xand the carbon atom substitutes a nitrogen atom adjacent to the atom X.In other words, the carbon atom and the second atom form a complex witha pair structure. As a back bond, the carbon atom forms three bonds withthe atom X. In addition, as a back bond, the second atom forms threebonds with the nitrogen atom.

The bond between the atom X and the carbon atom, the bond between thenitrogen atom and the second atom, and a complex with a pair structureof the carbon atom and the second atom in the high-resistance region 16b can be measured by, for example, X-ray photoelectron spectroscopy,infrared spectroscopy, or Raman spectroscopy.

The carbon concentration of the high-resistance region 16 b is, forexample, equal to or greater than 1×10¹⁹ cm⁻³. The concentration of thesecond atom in the high-resistance region 16 b is, for example, equal toor greater than 1×10¹⁹ cm⁻³. The concentration of carbon and the secondatom in the high-resistance region 16 b can be measured by, for example,secondary ion mass spectroscopy.

For example, the semiconductor device according to this embodiment canbe manufactured by implanting the second atoms into the barrier layer16, using ion implantation, at the same time as carbon ions areimplanted into the barrier layer 16 in the semiconductor devicemanufacturing method according to the first embodiment.

For example, when the atom X is a gallium (Ga) atom and the second atomis a silicon (S) atom, three gallium atoms are bonded to one carbon atomand three nitrogen atoms are bonded to one silicon atom. The carbon atomand the silicon atom form a complex with a pair structure. When the atomX is an aluminum (Al) atom, three aluminum atoms are bonded to onecarbon atom and three nitrogen atoms are bonded to one silicon atom. Thecarbon atom and the silicon atom form a complex with a pair structure.

FIGS. 17A and 17B are diagrams illustrating the function of thesemiconductor device according to this embodiment. FIG. 17A is a diagramschematically illustrating a silicon atom and a carbon atom in galliumnitride. FIG. 17B is a diagram illustrating a level when the carbon atomand the silicon atom calculated by the first principle calculationcoexist.

As illustrated in FIG. 17A, a silicon atom in gallium nitride is presentat the lattice position of a gallium atom in gallium nitride. A carbonatom in gallium nitride is present at the lattice position of a nitrogenatom in gallium nitride. FIG. 17A illustrates a state in which a siliconatom and a carbon atom are closest to each other when the silicon atomand the carbon atom coexist in gallium nitride. In other words, thesilicon atom and the carbon atom form a complex with a pair structure.

The first principle calculation proves that, when a silicon atom and acarbon atom coexist, a structure in which an electron moves from thedonor level of the silicon atom to the acceptor level of the carbon atomis stabilized, as illustrated in FIG. 17B. At that time, the levelformed by the silicon atom is moved to a conduction band and the levelformed by the carbon atom is moved to a valence band. Therefore, thelevel in the bandgap of gallium nitride is removed, as illustrated inFIG. 17B.

In this embodiment, a silicon atom is provided at the position of agallium atom and a carbon atom is provided at the position of a nitrogenatom in gallium nitride. The level in the bandgap of gallium nitride isremoved by the interaction between the silicon atom and the carbon atom.Therefore, electron trap is prevented. As a result, it is possible toprevent current collapse.

The removal of the level in the bandgap of gallium nitride can bedetermined by, for example, DLTS.

When a silicon atom and a carbon atom coexist, the donor level and theacceptor level are removed. As a result, carriers are cancelled andcarrier concentration is reduced. Therefore, the electric resistivity ofgallium nitride increases.

Since electron trap immediately below the gate electrode 28 isprevented, a variation in the threshold voltage is prevented.

VN is formed in gallium nitride in order to increase entropy. When acomplex with a pair structure of a carbon atom and a silicon atom isformed by carbon and silicon ion implantation and a sufficient heattreatment, entropy increases. Therefore, theoretically, when the amountof complex with a pair structure is sufficient, VN is removed. When VNremains, there is a concern that current collapse and a variation in thethreshold voltage will occur due to the donor level in the bandgapcaused by the remaining VN.

For example, it is assumed that the amount of carbon more than theamount of VN in gallium nitride is introduced by carbon ionimplantation. At that time, it is assumed that the amount of siliconmore than the amount of carbon is introduced by ion implantation. Inthis case, theoretically, VN is removed. However, surplus siliconremains. There is a concern that current collapse and a variation in thethreshold voltage will occur due to the donor level in the bandgapcaused by the remaining silicon.

For example, it is assumed that the amount of carbon more than theamount of VN in gallium nitride is introduced by carbon ionimplantation. At that time, it is assumed that the amount of siliconless than the amount of carbon is introduced by ion implantation.

In this case, it is assumed that, since the amount of carbon is morethan the amount of VN, once VN is removed. Then, the same amount of VNas the amount of carbon which remains without forming a pair structurewith silicon is generated in order to form a complex with carbon. Inother words, VN for compensating for surplus carbon which does not forma pair structure with silicon is newly generated. Therefore, the amountof carbon atoms, silicon atoms, or VN which remains independently ingallium nitride is very small.

In this case, when the amount of silicon is less than the amount of VN,a sufficiently large amount of carbon is introduced. Therefore, surplusVN forms a complex with a carbon atom and no VN remains independently.Then, the same amount of VN as the amount of carbon which remainswithout forming a complex with silicon is newly generated in order toform a complex with carbon. Therefore, the amount of carbon atoms,silicon atoms, or VN which remains independently in gallium nitride isvery small.

For example, it is assumed that the amount of carbon less than theamount of VN in gallium nitride is introduced by carbon ionimplantation. At that time, it is assumed that the amount of siliconmore than the amount of carbon is introduced by ion implantation. Inthis case, theoretically, VN corresponding to a shortage of carbonremains independently. In addition, surplus silicon remainsindependently. There is a concern that current collapse and a variationin the threshold voltage will occur due to the donor level in thebandgap caused by VN and silicon which remain independently.

For example, it is assumed that the amount of carbon less than theamount of VN in gallium nitride is introduced by carbon ionimplantation. At that time, it is assumed that the amount of siliconless than the amount of carbon is introduced by ion implantation. Inthis case, no silicon remains independently and VN corresponding to ashortage in carbon remains independently. There is a concern thatcurrent collapse and a variation in the threshold voltage will occur dueto the donor level in the bandgap caused by VN which remainsindependently.

Therefore, it is preferable that the concentration of carbon in galliumnitride be higher than the concentration of VN before carbon or siliconis introduced, in order to prevent current collapse and a variation inthe threshold voltage. In addition, it is preferable that theconcentration of carbon in gallium nitride be higher than theconcentration of silicon in gallium nitride.

It is considered that the concentration of VN in gallium nitride isequal to or greater than about 1×10¹⁸ cm⁻³ and equal to or less thanabout 1×10¹⁹ cm⁻³. The carbon concentration of the high-resistanceregion 16 b is preferably equal to or greater than 1×10¹⁹ cm⁻³, morepreferably equal to or greater than 1×10²⁰ cm⁻³, and most preferablyequal to or greater than 1×10²¹ cm⁻³, in order to prevent the generationof surplus VN. Since the amount of carbon is more than the amount of VNin gallium nitride, it is possible to control the level in the bandgap.

In this embodiment, instead of the silicon atom, an atom of germanium(Ge), titanium (Ti), zirconium (Zr), hafnium (Hf), or iron (Fe) may beused as the second atom. In this case, the same effect as that when thesecond atom is a silicon atom is obtained.

An energy gain obtained when a carbon atom and the second atom (Si, Ge,Ti, Zr, Hf, or Fe) are close to each other is higher than that obtainedwhen a carbon atom and VN are close to each other and high stability isobtained. The reason is that, when atoms are close to each other, it ispossible to ensure the localization of charge and coulomb interaction isstrong, as compared to when an atom and VN are close to each other. Inaddition, the second atom can move to the position where the second atomdirectly forms a bond with a carbon atom and is much closer to a carbonatom than VN or the first atom. Therefore, coulomb force furtherincreases.

The amount of second atoms or the distribution thereof can be controlledby ion implantation conditions. Therefore, it is easy to perform controlsuch that the amount of carbon is substantially equal to the amount ofsecond atoms or the distribution of carbon is equal to the distributionof the second atoms. Similarly to the first embodiment, for the secondatom, a sufficient amount of carbon is introduced and a shortage of thesecond atoms can be supplemented with VN generated by a high-temperatureheat treatment.

According to the semiconductor device and the semiconductor devicemanufacturing method of this embodiment, it is possible to preventcurrent collapse, similarly to the first embodiment. In addition, it ispossible to achieve a semiconductor device with high reliability. It ispossible to achieve a semiconductor device with a higher stability thanthat in the first and second embodiments. Furthermore, controllabilityduring manufacture is higher than that in the first embodiment.

Fourth Embodiment

A semiconductor device according to this embodiment differs from thesemiconductor device according to the first to third embodiments in thatit includes a nitride semiconductor layer, an insulating layer providedon the nitride semiconductor layer, and a region which is provided in aninsulating-layer-side portion of the nitride semiconductor layer andincludes an atom Y having a dangling bond, an atom Y forming a bond witha first atom of at least one element selected from the group consistingof oxygen (O), sulfur (S), selenium (Se), and tellurium (Te), or an atomY forming a bond with a second atom of at least one element selectedfrom the group consisting of silicon (Si), germanium (Ge), titanium(Ti), zirconium (Zr), hafnium (Hf), and iron (Fe). The atom Y is an atomof at least one element selected from the group consisting of beryllium(Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), zinc(Zn), and cadmium (Cd).

The semiconductor device according to this embodiment differs from thesemiconductor device according to the first embodiment using thenitrogen defect (VN) in that it includes a region in which the atom Yand VN form a complex with a pair structure.

Alternatively, the semiconductor device according to this embodimentdiffers from the semiconductor device according to the second embodimentusing the first atom in that it includes a region in which the atom Yand the first atom of at least one element selected from the groupconsisting of oxygen (O), sulfur (S), selenium (Se), and tellurium (Te)form a complex with a pair structure.

Alternatively, the semiconductor device according to this embodimentdiffers from the semiconductor device according to the third embodimentusing the second atom in that it includes a region in which the atom Yand the second atom of at least one element selected from the groupconsisting of silicon (Si), germanium (Ge), titanium (Ti), zirconium(Zr), hafnium (Hf), and iron (Fe) form a complex with a pair structure.

In the first to third embodiments, the carbon atom substitutes thenitrogen atom and functions as an acceptor. However, in this embodiment,the atom Y substitutes, for example, a gallium atom and functions as anacceptor. This point is the difference from the first to thirdembodiments. That is, an acceptor formation mechanism is different fromthat in the first to third embodiments.

The semiconductor device according to this embodiment differs from thesemiconductor device according to the first to third embodiments in thatit includes the atom Y at the position of the atom X which is an atomother than a nitrogen atom forming the crystal structure of the nitridesemiconductor layer, instead of the carbon atom in the first to thirdembodiments. Hereinafter, the description of the same content as that inthe first to third embodiments will not be repeated.

In this embodiment, the same function and effect as those in the firstto third embodiments are obtained. That is, VN, the first atom, or thesecond atom which functions as a donor and the atom Y which functions asan acceptor form a complex with a pair structure. Therefore, a level inthe bandgap of the silicon carbide layer is removed. In other words, thelevel in the bandgap of the silicon carbide layer is removed by theinteraction between VN, the first atom, or the second atom functioningas a donor and the atom Y functioning as an acceptor.

FIGS. 18A, 18B, and 18C are diagrams illustrating the semiconductordevice according to this embodiment. FIG. 18A is a diagram illustratinga first example, FIG. 18B is a diagram illustrating a second example,and FIG. 18C is a diagram illustrating a third example.

Next, the first example will be described.

The high-resistance region (region) 16 b illustrated in FIG. 1 includesan atom Y which is an atom of at least one element selected from thegroup consisting of beryllium (Be), magnesium (Mg), calcium (Ca),strontium (Sr), barium (Ba), zinc (Zn), and cadmium (Cd) and is presentat the lattice position of the atom X. For example, one atom Y isintroduced to the lattice position of a gallium atom in aluminum galliumnitride. The one atom Y introduced to the lattice position of thegallium atom functions as an acceptor.

A nitrogen defect (VN) is present in the high-resistance region 16 b.The nitrogen defect (VN) functions as a donor.

As illustrated in FIG. 18A, for example, it is assumed that the atom Xis a gallium (Ga) atom and the atom Y is a magnesium (Mg) atom. Themagnesium atom substitutes the gallium atom and VN and the magnesiumatom are adjacent to each other and form a complex with a pairstructure. In other words, the magnesium atom has a dangling bond.

According to the first example, a level in the bandgap of a nitridesemiconductor is removed by the same function as that in the firstembodiment. Therefore, electron trap is prevented. As a result, it ispossible to prevent current collapse and a variation in the thresholdvoltage.

In the first example, instead of the magnesium atom, an atom ofberyllium (Be), calcium (Ca), strontium (Sr), barium (Ba), zinc (Zn), orcadmium (Cd) which is a group-II element and is a bivalent element maybe used as the atom Y. In this case, the same effect as that when theatom Y is a magnesium atom is obtained.

Next, a second example will be described.

The high-resistance region (region) 16 b illustrated in FIG. 1 includesan atom Y which is an atom of at least one element selected from thegroup consisting of beryllium (Be), magnesium (Mg), calcium (Ca),strontium (Sr), barium (Ba), zinc (Zn), and cadmium (Cd) and is presentat the lattice position of the atom X. For example, one atom Y isintroduced to the lattice position of a gallium atom in aluminum galliumnitride. The one atom Y introduced to the lattice position of thegallium atom functions as an acceptor.

In addition, the high-resistance region 16 b includes a first atom of atleast one element selected from the group consisting of oxygen (O),sulfur (S), selenium (Se), and tellurium (Te). The first atom is presentat the lattice position of a nitrogen atom. The first atom functions asa donor.

As illustrated in FIG. 18B, for example, it is assumed that the atom Xis a gallium (Ga) atom, the atom Y is a magnesium (Mg) atom, and thefirst atom is an oxygen (O) atom. The magnesium atom substitutes thegallium atom and the oxygen atom substitutes a nitrogen atom adjacent tothe magnesium atom. Then, the magnesium atom and the oxygen atom arebonded to each other to form a complex with a pair structure.

According to the second example, a level in the bandgap of a nitridesemiconductor is removed by the same function as that in the secondembodiment. Therefore, electron trap is prevented. As a result, it ispossible to prevent current collapse and a variation in the thresholdvoltage.

In the second example, instead of the magnesium atom, an atom ofberyllium (Be), calcium (Ca), strontium (Sr), barium (Ba), zinc (Zn), orcadmium (Cd) which is a group-II element and is a bivalent element maybe used as the atom Y. In this case, the same effect as that when theatom Y is a magnesium atom is obtained.

In the second example, instead of the oxygen atom, an atom of sulfur(S), selenium (Se), or tellurium (Te) which is a group 16 element and isa bivalent element may be used as the first atom. In this case, the sameeffect as that when the first atom is an oxygen atom is obtained.

Next, a third example will be described.

The high-resistance region (region) 16 b illustrated in FIG. 1 includesan atom Y which is an atom of at least one element selected from thegroup consisting of beryllium (Be), magnesium (Mg), calcium (Ca),strontium (Sr), barium (Ba), zinc (Zn), and cadmium (Cd) and is presentat the lattice position of the atom X. For example, one atom Y isintroduced to the lattice position of a gallium atom in aluminum galliumnitride. The one atom Y introduced to the lattice position of thegallium atom functions as an acceptor.

In addition, the high-resistance region 16 b includes a second atom ofat least one element selected from the group consisting of silicon (Si),germanium (Ge), titanium (Ti), zirconium (Zr), hafnium (Hf), and iron(Fe). The second atom is present at the lattice position of the atom X.The second atom functions as a donor.

As illustrated in FIG. 18C, for example, it is assumed that the atom Xis a gallium (Ga) atom, the atom Y is a magnesium (Mg) atom, and thesecond atom is a silicon (Si) atom. The magnesium atom and the siliconatom substitute gallium atoms. The magnesium atom and the silicon atomare bonded to each other to form a complex with a pair structure.

According to the third example, a level in the bandgap of a nitridesemiconductor is removed by the same function as that in the thirdembodiment. Therefore, electron trap is prevented. As a result, it ispossible to prevent current collapse and a variation in the thresholdvoltage.

In the third example, instead of the magnesium atom, an atom ofberyllium (Be), calcium (Ca), strontium (Sr), barium (Ba), zinc (Zn), orcadmium (Cd) which is a group-II element and is a bivalent element maybe used as the atom Y. In this case, the same effect as that when theatom Y is a magnesium atom is obtained.

In the third example, instead of the silicon atom, an atom of germanium(Ge), titanium (Ti), zirconium (Zr), hafnium (Hf), or iron (Fe) may beused as the second atom. In this case, the same effect as that when thesecond atom is a silicon atom is obtained.

According to the semiconductor device of this embodiment, it is possibleto prevent current collapse, similarly to the first to thirdembodiments. In addition, it is possible to achieve a semiconductordevice with high reliability.

Fifth Embodiment

A semiconductor device according to this embodiment is the same as thesemiconductor device according to the first embodiment except that thep-type layer comes into contact with the nitride semiconductor layer.Therefore, the description of the same content as that in the firstembodiment will not be repeated.

FIG. 19 is a cross-sectional view schematically illustrating thesemiconductor device according to this embodiment. The semiconductordevice according to this embodiment is an HEMT using a GaN-basedsemiconductor.

In an HEMT (semiconductor device) 200, a p-type layer 24 is provided soas to come into contact with the barrier layer 16. The p-type layer 24comes into contact with the low-resistance region 16 a. The p-type layer24 is, for example, a single-crystal gallium nitride (GaN) layer.

According to the semiconductor device of this embodiment, similarly tothe first embodiment, a level in the bandgap is reduced to preventcurrent collapse. In addition, the level in the bandgap is reduced toprevent a variation in the threshold voltage. Therefore, it is possibleto achieve a semiconductor device with high reliability.

Sixth Embodiment

A semiconductor device according to this embodiment is the same as thesemiconductor device according to the first embodiment except that thegate electrode comes into contact with the nitride semiconductor layer.Therefore, the description of the same content as that in the firstembodiment will not be repeated.

FIG. 20 is a cross-sectional view schematically illustrating thesemiconductor device according to this embodiment. The semiconductordevice according to this embodiment is an HEMT using a GaN-basedsemiconductor.

In an HEMT (semiconductor device) 300, agate electrode 28 is provided soas to come into contact with the barrier layer 16. The gate electrode 28comes into contact with the low-resistance region 16 a.

The gate electrode 28 is, for example, a metal electrode. The gateelectrode 28 is made of, for example, titanium nitride (TiN).

The junction between the gate electrode 28 and the barrier layer 16 is aSchottky junction. The HEMT 300 is a normally-on transistor.

According to the semiconductor device of this embodiment, similarly tothe first embodiment, a level in the bandgap is reduced to preventcurrent collapse. In addition, the level in the bandgap is reduced toprevent a variation in the threshold voltage. Therefore, it is possibleto achieve a semiconductor device with high reliability.

Seventh Embodiment

A semiconductor device according to this embodiment is the same as thesemiconductor device according to the first embodiment except that ithas a so-called gate recess structure in which a gate electrode isburied in a recess formed in a barrier layer. Therefore, the descriptionof the same content as that in the first embodiment will not berepeated.

FIG. 21 is a cross-sectional view schematically illustrating thesemiconductor device according to this embodiment.

In an HEMT (semiconductor device) 400, a high-resistance region 14 b andthe insulating layer 22 are formed on the inner surface of a recess 30that is provided in the barrier layer 16 between the source electrode 18and the drain electrode 20. In addition, the high-resistance region 16 bis provided.

The bottom of a recess 30 is located in the channel layer 14. Thehigh-resistance region 14 b provided at the bottom of the recess 30 isformed in the channel layer 14. A low-resistance region 14 a and thehigh-resistance region 14 b form the channel layer 14.

According to the semiconductor device of this embodiment, a level in thebandgap is reduced to prevent current collapse, similarly to the firstembodiment. In addition, the level in the bandgap is reduced to preventa variation in the threshold voltage. Therefore, it is possible toachieve a semiconductor device with high reliability. In addition, sincethe semiconductor device has the gate recess structure, it is easy toachieve a normally-off transistor.

Eighth Embodiment

A semiconductor device according to this embodiment differs from thesemiconductor device according to the seventh embodiment in that thebarrier layer is provided below the recess. Hereinafter, the descriptionof the same content as that in the seventh embodiment will not berepeated.

FIG. 22 is a cross-sectional view schematically illustrating thesemiconductor device according to this embodiment.

In an HEMT (semiconductor device) 500, the barrier layer 16 is providedat the bottom of the recess 30. The barrier layer 16 is provided on thechannel layer 14. A protective layer 17 made of nitride semiconductor isprovided on both sides of the recess 30. The protective layer 17 isformed on the barrier layer 16 by, for example, selective epitaxialgrowth.

The channel layer 14 is made of, for example, GaN. The barrier layer 16is, for example, an Al_(0.1)Ga_(0.9)N layer with a thickness of 10 nm.The protective layer 17 is, for example, an Al_(0.2)Ga_(0.8)N layer witha thickness of 20 nm.

In the HEMT 500, the high-resistance region 16 b and the insulatinglayer 22 are formed on the inner surface of the recess 30. Thelow-resistance region 16 a and the high-resistance region 16 b form thebarrier layer 16.

The insulating layer 22 is also formed between the source electrode 18and the drain electrode 20. In addition, a low-resistance region 17 aand a high-resistance region 17 b are provided between the barrier layer16 and the insulating layer 22.

The HEMT 500 is a normally-off transistor. A back barrier layer (notillustrated) which is made of a GaN-based semiconductor and has a higherbandgap than the channel layer 14 may be provided in at least a portionbetween the buffer layer 12 and the channel layer 14 in order toincrease the threshold voltage of the HEMT 500. The back barrier layeris made of, for example, Al_(0.1)Ga_(0.9)N. The back barrier layer maybe doped with, for example, Mg such that it is a p type.

In addition, a p-type layer (not illustrated) made of a p-type GaN-basedsemiconductor may be provided at the bottom of the recess 30 in order toincrease the threshold voltage of the HEMT 500. The p-type layer is madeof, for example, p-type GaN.

According to the semiconductor device of this embodiment, similarly tothe first embodiment, a level in the bandgap is reduced to preventcurrent collapse. In addition, the level in the bandgap is reduced toprevent a variation in the threshold voltage. Therefore, it is possibleto achieve a semiconductor device with high reliability.

Ninth Embodiment

A power circuit and a computer according to this embodiment include anHEMT.

FIG. 23 is a diagram schematically illustrating the computer accordingto this embodiment. The computer according to this embodiment is aserver 600.

The server 600 includes a power circuit 42 provided in a housing 40. Theserver 600 is a computer that executes server software.

The power circuit 42 includes the HEMT 100 according to the firstembodiment. Instead of the HEMT 100, the HEMT 200, the HEMT 300, theHEMT 400, and the HEMT 500 according to the fifth to eighth embodimentsmay be applied. The power circuit 42 may be a power circuit forautomobile use.

Since the power circuit 42 includes the HEMT 100 in which currentcollapse is prevented, it has high reliability. Since the server 600includes the power circuit 42, it has high reliability.

According to this embodiment, it is possible to achieve a power circuitand a computer with high reliability.

In the above-described embodiments, GaN or AlGaN is given as an exampleof the material forming the GaN-based semiconductor layer. However, forexample, InGaN, InAlN, and InAlGaN including indium (In) may be applied.In addition, AlN may be applied as the material forming the GaN-basedsemiconductor layer.

In the above-described embodiments, the invention is applied to theHEMT. However, the invention is not limited to the HEMT and may beapplied to other devices such as transistors or diodes.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the semiconductor device, the powercircuit, and the computer described herein may be embodied in a varietyof other forms; furthermore, various omissions, substitutions andchanges in the form of the devices and methods described herein may bemade without departing from the spirit of the inventions. Theaccompanying claims and their equivalents are intended to cover suchforms or modifications as would fall within the scope and spirit of theinventions.

1-20. (canceled) 21: A semiconductor device comprising: a nitridesemiconductor layer comprising a first layer and a second layer; and aninsulating layer provided on the nitride semiconductor layer, wherein:the second layer is provided between the first layer and the insulatinglayer, the second layer has a higher electric resistivity than the firstlayer, and the second layer comprises an atom Y, the atom Y being anatom of at least one element selected from the group consisting ofberyllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium(Ba), zinc (Zn), and cadmium (Cd). 22: The semiconductor deviceaccording to claim 21, wherein a concentration of the atom Y in thesecond layer is equal to or greater than 1×10¹⁹ cm⁻³. 23: Thesemiconductor device according to claim 21, wherein the nitridesemiconductor layer comprises a first semiconductor region and a secondsemiconductor region having a wider bandgap than the first semiconductorregion, and the first layer and the second layer are provided in thesecond semiconductor region. 24: The semiconductor device according toclaim 23, wherein the first semiconductor region is made of galliumnitride and the second semiconductor region is made of aluminum galliumnitride. 25: The semiconductor device according to claim 21, wherein thenitride semiconductor layer comprises gallium (Ga). 26: Thesemiconductor device according to claim 21, wherein the second layercomprises a first atom, the first atom being at least one elementselected from the group consisting of oxygen (O), sulfur (S), selenium(Se), and tellurium (Te). 27: The semiconductor device according toclaim 21, wherein the second layer comprises a second atom, the secondatom being at least one element selected from the group consisting ofsilicon (Si), germanium (Ge), titanium (Ti), zirconium (Zr), hafnium(Hf), and iron (Fe). 28: The semiconductor device according to claim 21,wherein, when an atom other than a nitrogen atom forming a crystalstructure of the nitride semiconductor layer is an atom X, the atom Y ispresent at a lattice position of the atom X. 29: A semiconductor devicecomprising: a nitride semiconductor layer comprising a first layer and asecond layer; an insulating layer provided on the nitride semiconductorlayer; a first electrode contacting the first layer; a second electrodecontacting the first layer; and a gate electrode provided between thefirst electrode and the second electrode; wherein: the second layer isprovided between the first layer and the insulating layer, the secondlayer has a higher electric resistivity than the first layer, and thesecond layer comprises an atom Y, the atom Y being an atom of at leastone element selected from the group consisting of beryllium (Be),magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), zinc (Zn),and cadmium (Cd). 30: The semiconductor device according to claim 29,wherein the first electrode directly contacts the first layer, and thesecond electrode directly contacts the first layer. 31: Thesemiconductor device according to claim 29, wherein a concentration ofthe atom Y in the second layer is equal to or greater than 1×10¹⁹ cm⁻³.32: The semiconductor device according to claim 29, wherein the secondlayer comprises a first atom, the first atom being at least one elementselected from the group consisting of oxygen (O), sulfur (S), selenium(Se), and tellurium (Te). 33: The semiconductor device according toclaim 29, wherein the second layer comprises a second atom, the secondatom being at least one element selected from the group consisting ofsilicon (Si), germanium (Ge), titanium (Ti), zirconium (Zr), hafnium(Hf), and iron (Fe). 34: The semiconductor device according to claim 29,wherein, when an atom other than a nitrogen atom forming a crystalstructure of the nitride semiconductor layer is an atom X, the atom Y ispresent at a lattice position of the atom X. 35: A semiconductor devicecomprising: a nitride semiconductor layer comprising a first layer and asecond layer; and an insulating layer provided on the nitridesemiconductor layer; wherein: the second layer is provided between thefirst layer and the insulating layer, the second layer comprises atleast one atom selected from a first atom Y, a second atom Y, and athird atom Y, the first atom Y having a dangling bond, the second atom Yforming a bond with a first atom of at least one element selected fromthe group consisting of oxygen (O), sulfur (S), selenium (Se), andtellurium (Te), the third atom Y forming a bond with a second atom of atleast one element selected from the group consisting of silicon (Si),germanium (Ge), titanium (Ti), zirconium (Zr), hafnium (Hf), and iron(Fe), and atom Y being an atom of at least one element selected from thegroup consisting of beryllium (Be), magnesium (Mg), calcium (Ca),strontium (Sr), barium (Ba), zinc (Zn), and cadmium (Cd). 36: Thesemiconductor device according to claim 35, wherein, when an atom otherthan a nitrogen atom forming a crystal structure of the nitridesemiconductor layer is an atom X, the first atom Y, the second atom Y,and the third atom Y is present at a lattice position of the atom X. 37:The semiconductor device according to claim 36, wherein the atom X is agallium (Ga) atom or an aluminum (Al) atom. 38: A power circuitcomprising: the semiconductor device according to claim
 21. 39: Acomputer comprising: the semiconductor device according to claim 21.